Shear Adhesion Failure Temperature SAFT

Of the various methods used to manufacture UV-curable PSA, that of preparing a UV-crosslinkable acrylic PSA was adapted to evaluate the UV-curing behavior and adhesion performance at various UV doses. The UV-crosslinkable PSAs were copolymerized with acrylate monomers and an unsaturated photoinitiator, P-36, with varying contents of 2-HEMA and photoinitiator. 

Satas (Ed.), Handbook of Pressure Sensitive Adhesive Technology and Applications, Satas Associates, Warwick, 2002, Chapter 4. 

Mechanisms of Photophysical Processes and Photochemical Reactions in Polymers, John Wiley Sons, Chichester, 1987, Chapters 7 and 11. 

Roffey Photogeneration of Reactive Species for UV Curing, J ohn Wiley Sons, Chichester, 1997, Chapter 5. 

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EHA), vinyl acetate (VAc) and acrylic acid (AA), and with varying contents of 2-hydroxyefhyl methacrylate (2-HEMA) and an unsaturated benzophenone derivative (P-36). The UV-crosslinking behavior of the PSAs was monitored by FTIR-ATR and PSA performance was evaluated by probe tack, peel strength, and shear adhesion failure temperature (SAFT) after exposure to various UV doses. 

One method used to evaluate the upper working temperature of block copolymer systems is to measure the shear adhesion failure temperature (SAFT), a useful method to discover exactly what has been gained in the upper working temperature limits when resins are added to the styrene domain of polystyrene end-block systems to increase the effective glass transition temperature. Typically the test is set up as a standard shear test, either to the standard stainless steel panel or to polyester, with a 1 in. by 1 in. (25 mm by 25 mm) area and a load of 1 kg. The set-up is placed in an oven that can be accurately controlled so that the temperature is increased by 2.0°C (3.6°F) per minute, the temperature at which failure occurs being recorded as the SAFT. 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

Shear adhesion failure temperature (SAFT) gives information on the resistance of the PSA to stress at elevated temperature. The geometry is identical to the shear adhesion test (Fig. 13). The test is performed in a forced-air oven with temperature rising at constant rate (0.4 °C/min). The SAFT is the temperature at which the bond fails and the load falls down. As this method is not standardized yet, there are some differences in overlap and load between companies [225], [226]. 

Polybutylene polymers slow oystallization rates are helpful in forming hot-melt adhesives with long open times [67]. Besides the hot-melt viscosity, lap shear strength, T-peel, and open time, the shear adhesion failure temperature (SAFT) of the bonded substrate is an inqxntant noperty for evaluating the effectiveness of the adhesive. The polymer MFI plays an inqxntant part in determining the surface tenqterature at whidi shear adhesion fidlure could occur, as can be seen from Fig. 9.75. The top curve represents the SAFT test results with a 0.5-kg load, whereas the bottom curve shows test results with a 1-kg... 

Shear Adhesion Failure Temperature (SAFT) Temperature at which complete failure of a 1 in. x 1 in. shear adhesion sample (polyester to polyester) occurs under a load of 1 kg in a circulating air chamber whose temperature is raised 40°F per hour starting at 1(X)°F. 

There is second shear adhesion test that is now being reported with increasing frequency, the so-called SAFT, or shear adhesion failure temperature test. It is particularly popular among block copolymer PSA developers. In this test [15], a shear specimen with an overlap area of 2.54 cm x 2.54 cm is prepared and suspended in a circulating air oven. A 1-kg weight is attached to the tape and the oven temperature is raised continuously 5.5°C per 15 min until failure. An industry wide standard has not yet been written for this test. 

Another type of shear test involves testing under heated conditions. The shear adhesion failure test (SAFT) (FINAT FTM 8, PSTC-7) is a static shear test administered to a PSA sample under increasing temperature at a rate typically of 1 or 2°C per minute. The recorded SAFT value is the temperature at which the adhesive bond fails. 

As with other pressure-sensitive adhesive tests, the temperature is an important variable and tests are typically carried out at room temperature. A variation on this method is the SAFT test (shear-adhesion-failure temperature) in which the test apparatus is placed in an oven where the temperature is set to rise at 4.5°C/min. The temperature at which the tape fails is recorded as the SAFT value. 

Since creep is considered as a key weaknesses of pressure sensitive adhesives, various specific test methods and standards have been developed to evaluate the creep resistance of pressure sensitive adhesives like, for example, in the European Standard EN 1943 ( Self adhesive tapes-Measurement of static shear adhesion ), FINAT (Federation Internationale des Fabricants et Transformateurs d Adhesives et ThermocoUants sur Papier et autres Supports), test method FTM 8 (Resistance to shear from a standard surface), or the Pressure Sensitive Tape Councils test method PSTC 107 (International Standard for Shear Adhesion of Pressure Sensitive Tape) by either monitoring the time- and load-dependent displacement of an adhesive specimen under shear load or simply recording the time to failure. The result of the so called SAFT-test ( Shear Adhesion Failure Temperature ) indicates the temperature at which a sample that has been subjected to an environment with steadily rising temperature under static shear load has failed. 

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